Rigid disc drive data storage devices of the type in which the present invention is particularly useful are well known in the industry. Such devices incorporate a base housing to which is mounted a spindle motor, usually of the brushless DC type, having a hub supporting at least one data storage disc. Each disc carries a number of circular, concentric data tracks on which data can be stored and from which the data can later be retrieved. The processes of storing and retrieving data are usually referred to as "writing" and "reading", respectively, and are accomplished using read/write heads. There is usually one read/write head associated with each surface of each disc.
These read/write heads are incorporated in a slider assembly that is shaped to allow the heads to "fly" above the surface of the disc on a very thin layer of air dragged along by the spinning of the disc. During normal read/write operations, therefore, the heads and discs never come into contact.
In order for the single head to access the multiple tracks on the disc surface, the head is mounted on a load beam/gimbal assembly (LGA) which is in turn attached to some sort of actuator.
Disc drive actuators are generally one of two types: linear or rotary.
The linear actuator moves the heads on a straight line which is a radius of--or parallel to a radius of--the disc, guided by an arrangement of ball bearings or bushings with guide rails or rods.
The rotary actuator pivots about a single point closely adjacent the outer diameter of the disc and moves the heads along an arc--centered on the pivot--which crosses all of the data tracks.
Whichever type of actuator is used to move the heads, some sort of motor is incorporated to power the motion. Again, there are two commonly used motor types: stepper motors and voice coil motors (VCMs). The primary function of whichever motor is used is to move the heads from track to track in a controlled fashion under the control of electronic circuitry.
A second function of the actuator motor is to move the read/write heads to a preselected "park" position when a loss of power is detected. This is usually done to ensure that the heads do not come into direct contact with the surface of the disc where data has been written when the discs stop spinning and the heads stop "flying".
With stepper motor actuators, the detent magnetism of the stepper motor is usually adequate to hold the heads in the parked position. However, VIM actuators have no inherent detent to hold the heads in the parked position, and therefore must incorporate some sort of latch mechanism.
The power to park the heads at the time of a power loss is typically derived from the back EMF of the spindle motor, i. e., the spindle motor continues to spin after a loss of power due to inertia, and acts as a generator. Simply applying a DC current to the coil of a VIM is sufficient to move the actuator to either end of is range of travel, dependent on the polarity of the current applied. This technology is well known in the art.
An example of using the back EMF of the spindle motor to park the heads in a stepper motor drive is disclosed in U.S. Pat. No. 4,679,102, issued Jul. 7, 1987 and assigned to the assignee of the present invention and incorporated herein by reference.
With the growing market in laptop and notebook personal computers, manufacturers of rigid disc drives have begun to use ramping devices for parking the heads. These ramping devices have the added advantage of preventing any direct contact between the discs and the read/ write heads, since, when power is applied to the disc drive, the heads remain parked on the ramps until the spindle motor has begun spinning fast enough to "fly" the heads. The heads are then carefully moved off the ramp structure onto an already established "air bearing" as the thin layer of rapidly spinning air is known.
Once again, with a stepper motor drive, the magnetic detent of the stepper motor is usually sufficient to "latch" the heads into the park position on ramps, while VIM drives--having no inherent detent--require some sort of latch mechanism to hold the heads in the park position.
Another consideration that must be made by the designer of a disc drive is the possibility of failure of the electronic circuitry controlling the movement of the actuator. Should such a failure occur, it is easy to imagine the actuator moving the heads in an uncontrolled manner to either end of the intended range of motion. If this uncontrolled motion is toward the inside of the disc, it could easily result in the heads contacting the hub that mounts the discs, resulting in fatal damage to the heads. Depending on the design, similar damage could result if the heads are moved outward on the disc with no control.
Also, as previously mentioned, the actuator is frequently intended to force the heads to a park position when power is lost, and this park position is usually at one extreme of the actuator range of motion. When ramps are not used, and the heads contact the disc in the park position, this park position is typically at the inner area of the disc to minimize the start torque requirement of the spindle motor. When ramping devices are used, the ramps--and therefore the park position--are located at the outside diameter of the discs.
Since these park operations occur at the loss of power, it would be unwise on the part of the designer to rely on electronic logic--powered by a deteriorating supply--to detect when the heads have been properly parked.
To limit the range of motion of the actuator and heads under these conditions, disc drive designers have long been known to include "crash stops" in their products. An example of crash stops is disclosed in U.S. U.S. Pat. No. 4,471,396, issued Sep. 11, 1984 and assigned to the assignee of the present invention and incorporated herein by reference.
Latches to hold the actuator of VIM disc drives in the park position are also well known in the art. Examples of typical latches are shown in U.S. Pat. Nos. 4,725,907, issued Feb. 16, 1988, 4,716,480, issued Dec. 29, 1987, and 4,890,176, issued Dec. 19, 1989, all assigned to the assignee of the present invention and incorporated herein by reference.
Frequently the crash stop and latch mechanisms are implemented as separate components, adding to the cost and complexity of the product. Furthermore, several typical latch mechanisms require a solenoid or similar powered device to release the actuator when power is applied, again adding cost and complexity and increasing power drain at power on.